This invention relates to improvements in a four high mill of a paired-roll-crossing type.
In recent years the demand for accuracy of thickness in the width direction of rolled metals has become more and more exacting. The demand has been met by initial crowning of rolls for rolling mills to make up for the deflections of the rolls under rolling load. This counter-measure necessitates having a large variety of initially crowned rolls in reserve, since the rolls must be replaced by rolls having a different crowning amount whenever the rolling condition, such as the width and/or the thickness of the slab or strip, are to be changed. Frequent roll changing lowers the rate of mill operation. Moreover, the crowning effect on rolls is highly variable because of the wear and thermal expansion of the rolls with the progress of the rolling operation. For these reasons there has been an incessant need in the art for means of controlling the thickness of the workpiece across the width of the rolling mill without the necessity of changing the rolls.
As a solution to this control problem, bending of the work rolls has been proposed and has proven to be fairly effective. However, the strengths of the work roll shafts place limits upon the bending forces applicable, and the roll-bending method is not satisfactorily capable of correcting thickness nonuniformity in the width direction of the workpiece. Especially since the strip width is relatively narrow for barrel length of the rolls, the roll bending effect cannot be fully achieved in most cases because of the interference by the contacting ends of the backup rolls and work rolls.
Therefore, the introduction of a rolling mill has been urgently called for which can freely control the thickness of the metal across the width thereof, according to changes in the given rolling conditions, while using the same rolls.
Another disadvantage of the prior art is the necessity of frequently replacing the mill rolls, especially the work rolls, due to rapid wear during rolling operations. Conventional mills are designed to receive slabs or strips in such a manner that the middle point of the roll length matches the center of the width of each passing piece. Consequently, the middle portions of the rolls wear faster than the end portions, and a common practice has been to remove each roll for replacement when its middle portion has worn beyond a predetermined limit. This means that the roll must be dismounted when its both ends are yet to reach the wear limits. The short life and frequent replacement of the rolls have lowered the rate of mill operation, and this has combined with the economical loss due to roll wear to have a serious effect upon the cost of the product.
Thus, with the view to eliminating the disadvantages of the conventional rolling mills, this applicant has already proposed novel four high mills, as disclosed in the specifications (with drawings) of copending Japanese Patent Application Nos. 138837/1978 and 59327/1979, which are capable of controlling the thickness extensively width-wise of the workpiece using the same rolls and more than doubling their ordinary spans of life.
Those rolling mills, of the four high type, include means for separately setting the left and right roll gaps, oil hydraulic jacks for roll bending, and a paired-roll-crossing mechanism which can be adjusted to move the pair of the upper work roll and backup roll and the pair of the lower work roll and backup roll to cross each other at predetermined angles with respect to a line normal to the rolling direction. In this way they can attain the end described above.
However, when the paired-roll-crossing mechanism is adjusted to turn the upper and lower pairs of rolls to the points where they cross at certain angles to a line normal to the rolling direction, there will occur horizontal deviations of the center points of the upper backup roll bearings and those of the reduction screws, away from each other. Bending moments corresponding to the amounts of deviations will develop in the screws, adversely affecting the screw operation. Similarly, the center points of the lower backup roll bearings and those of reduction cylinders undergo relative horizontal deviations, producing bending moments in the cylinders, too, and thereby having unfavorable effects upon the same.